Large oblong plastic tanks, such as those for containing water, or for containing wastewater in the particular case of septic tanks, have been fabricated by various means in the past, including by molding two dish-shape halves and then mating the halves at a lengthwise planar joint. U.S. Pat. No. 5,361,930 of Perry, U.S. Pat. No. 7,572,372 to Graf and U.S. Pat. No. 5,878,907 to Graf describe such kinds of tanks. Such type of tank construction provides an advantage over one piece plastic tanks in that half tanks may be nested for shipment and storage, potentially reducing handling, storage and transport costs. And it can provide an economic and ease-of-handling advantage over heavy concrete septic tanks that are also typically comprised of concave halves which are mated at the point of use.
Commercial Graf patent type of tanks have been made by injection molding, which produces good dimensional control and fidelity. That means better engineering use of plastic material than is achieved in less dimensionally precise tanks, such as are made by fiberglass resin lay-up or by blow molding or rotational molding. In one approach, described in the Graft patents, each injection molded tank half is identical, which lowers the cost of tooling.
The present invention relates to improvements useful with oblong shape injection molded half tanks that are later assembled into whole tanks, particularly septic tanks, either at the point of use, or more preferably a satellite assembly point, from which they are thereafter transported to the site of use where they are installed in a pit created in soil.
In the past, one piece oblong plastic septic tanks often have had molded lifting lugs at the top of the tank, to enable transport and installation at the point of use. For example see the four tabs which are at the top of a rotationally molded tank, described in Kruger U.S. Pat. No. 8,070,005. Handling half tanks to make them into whole tanks can present somewhat different problems which are not solved by use of the prior art teaching. The reason is that half tanks are shipped and stored as nested stacks; and in a typical assembly process, one half is placed on a work surface with the concave side facing upwardly, while the other half has to be placed concave-down on top of it. Since assembly of a two-piece tank will likely take place at place away from the factory, and the parts can be large and heavy, there is a need for aids which facilitate the handling and assembly of the molded half tanks in a safe way and in a way which avoids damage to the joint region.
There is another aspect relating to use of a buried septic tank that is assembled from molded half tanks, which is shared with one-piece plastic tanks. It is that a tank must first resist the load of overlying soil load as well as possible loads from motor vehicles and the like; and for such purpose plastic tanks have had corrugated walls for such purpose. See for example descriptions of septic tanks in U.S. Pat. Nos. 7,178,686, 7,144,506, 4,359,167, 5,361,930, and U.S. Pat. Pub. No. 2002/0029026.
Typically two, and at least one, large access ports are needed at the top of the tank. Often a riser or lid is attached to the top of the tank at the port location. Access ports can reduce the load bearing capacity of the top of a tank. And when as in the aforementioned Graf tanks, and in the tanks of the present invention, the top and bottom halves are identical, planar surfaces of tank access port regions, when forming part of the bottom of the assembled tank, will tend to be weaker than the adjacent corrugated tank walls.
There are certain loads associated with handling and testing that are somewhat unique to half tank technology. For example, some times assembled tanks are tested in the assembly shop, as by filling with water or by applying a vacuum. And of course an assembled tank has to be handled by transporting it to point of use typically an excavation in the earth. A tank made from half tanks has to resist the stresses associated with such handling and testing, and sometimes it has to meet regulatory criteria by not changing in dimension more than a defined amount, when the tank interior is partially evacuated.
When the half tank forms the top of the buried whole tank, there is a load which results from hold-down tethers which typically run across the top of a septic tank from opposing side dead-weight anchors, to hinder the tank from rising from the soil if it is emptied when the surrounding water table is high. See U.S. Pat. Nos. 7,028,967 and 6,786,689 for examples of such anchors.
When a half tank forms the bottom of a tank, and the tank is set on a hard flat surface and filled with water for testing, the stress which result can be quite different from those encountered during normal use when the tank is buried in soft soil.
Generally, adding strengthening features to a tank top or elsewhere undesirably increases material and molding cost. When a tank is made from identical molded half tanks, that can mean that features added to increase the strength of the half used as the top, to provide for handling, can be an unnecessary and wasteful presence in the half that is used as the bottom, thus making more important good design. Thus there is a continuing need for economic tank design.